This invention relates to circuits for controlled charging and discharging of energy storage devices such as capacitors associated with a load, and more particularly to a new and improved circuit for controlled charging and discharging of an energy storage device in response to predetermined fluid conductivity conditions.
One area of use of the present invention is in controlled firing of electro-explosive devices, although the principles of the invention can be variously applied. Electro-explosive devices can find use in a variety of applications, for example parachute canopy release mechanisms, pressurized gas release devices for inflating floatation equipment such as life vests or life rafts, and many other applications. A typical circuit for activating an electro-explosive device includes a capacitor which is charged from a supply and then discharged in a controlled manner through the device. The circuit also includes sensing electrodes and a conductivity sensing portion whereby the controlled charging and discharging of the capacitor typically has been performed only when the sensing electrodes are exposed to liquid having a predetermined electrical conductivity, i.e. a body of water.
U.S. Pat. No. 4,763,077 issued Aug. 9, 1988 and assigned to the assignee of the present invention discloses a circuit wherein the controlled charging and discharging of an energy storage device or capacitor is in response to sensing electrodes being exposed sequentially to mediums of different conductivity. In particular, the capacitor could be charged when the electrodes are exposed to a body of water and substantially discharged when the electrodes leave the water and are exposed to air. One example of use of that circuit is firing an electro-explosive device in a pressurized gas release device for inflating floatation equipment in the form of life vests. When a helicopter enters water it usually inverts, and to prevent drowning the life vest worn by the pilot should not be automatically inflated until he is able to leave the submerged helicopter and rise to the surface of the water. When ship personnel wearing life vests work below deck and a large volume of water suddenly enters the area, it is desired not to have the vests inflated so that they can more easily climb ladders or stairs in leaving the area. Another example is firing an electro-explosive device in an aviator helmet breathing mask release mechanism. It is desired not to release the masks while the pilot is under water after ejection and descent by parachute thereby utilizing the small residual air supply in the breathing mask and tube. However, once the pilot's head emerges above the water surface, it is desired to remove the mask especially since the pilot may be unconscious and drown if the mask is not removed.
An important consideration in the design of such a circuit is avoiding premature operation in response to rapid excursions in the apparent conductivity at the sensing electrodes caused by turbulence in the water. Another important design consideration is avoiding undesired operation in response to an apparent conductivity change when the electrodes are first exposed to salt fog spray and then become dry. Accordingly, it would be highly desirable to provide such a circuit wherein operation is in response to the sensing electrodes being exposed to separate or distinct mediums of different electrical conductivity, not merely apparent conductivity changes due to turbulence in one medium or to the effect of salt fog exposure.